Surface-emitting semiconductor lasers with external resonators, also known as disc lasers or VECSELs (Vertical External Cavity Surface Emitting Laser), are distinguished by a high output power together with high beam quality.
Such surface-emitting semiconductor lasers offer the possibility, in particular, of integrating into the laser resonator a means for frequency conversion of the emitted radiation. In this way it is especially possible, with a semiconductor material which emits radiation in the infrared spectral range, to use frequency doubling to generate visible light, in particular in the green spectral range. Frequency conversion in this case is generally brought about by a nonlinear optical crystal. To achieve efficient frequency conversion, the semiconductor laser should display a narrow-band emission spectrum and a high optical power within the laser resonator. It is especially desirable for the semiconductor laser to emit radiation of a single longitudinal mode.
To achieve single mode operation, two transmission filters in the form of etalons are often used in the external resonator.
As a rule, a first transmission filter is used in this case which has a free spectral range of such a size that the spectral overlap with the gain curve of the active material causes only the modes of one transmission maximum to oscillate in laser operation. Jumping of the laser modes between different transmission maxima of the filter is thus prevented. However, as a rule, the transmission maximum of a transmission filter in the form of an etalon with a large free spectral range generally has such a large full width at half maximum that a plurality of longitudinal modes of the laser resonator may start to oscillate within this transmission maximum. For this reason a second transmission filter is used, in order to select an individual longitudinal mode from the longitudinal modes of the laser resonator which fall into the transmission maximum of the first filter.
However, the use of two filters requires precise adjustment of the two filters and may generate undesired optical losses in the laser resonator.